pkg/cover/backend: fix arch usage #15779

Job	Run time
aux	4m 1s
arch (presubmit_arch_linux)	10m 7s
arch (presubmit_arch_freebsd)	6m 13s
arch (presubmit_arch_netbsd)	3m 49s
dashboard	2m 45s
gvisor	3m 35s
arch (presubmit_arch_openbsd)	3m 38s
build	4m 27s
arch (presubmit_arch_darwin)	3m 42s
old	6m 41s
arch (presubmit_arch_windows)	3m 28s
race	7m 45s
race_dashboard	4m 5s
arch (presubmit_arch_executor)	3m 10s
	1h 7m 26s

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